This disclosure of Japanese Patent Application No. 2001-367118 filed on Nov. 30, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an internal combustion engine. More specifically, the invention relates to an internal combustion engine capable of switching, during engine operation, between two combustion states in both of which combustion temperature and oxygen concentration differ greatly, respectively.
2. Description of Related Art
In general, various measures are taken with respect to a diesel engine, which is one type of a lean-burn internal combustion engine, so as to reduce emission of soot and nitrogen oxides (NOx). Such measures include, for example, a low-temperature combustion technology disclosed in the Japanese Patent Laid-Open Publication No. 2000-64911.
According to the low-temperature combustion technology disclosed in this Publication, a ratio of EGR gas contained in intake air subjected to combustion is controlled by increasing and reducing the amount of the EGR gas and air which flow into a combustion chamber, thereby enabling two engine combustion states, both of which combustion temperature and oxygen concentration differ greatly, respectively.
More specifically, in order to secure driveability during high-load operation, a normal combustion is carried out while regulating the ratio of the EGR gas contained in intake air to the appropriate value. On the other hand, during idling and low-load operation, the ratio of the EGR gas is significantly increased so as to significantly reduce the combustion temperature and the oxygen concentration, and the combustion state is switched into a so-called xe2x80x9clow-temperature combustionxe2x80x9d for reducing the amount of soot (smoke) and nitrogen oxides (NOx) generated.
Meanwhile, the diesel engine is operated under an excess air condition in which A/F reaches 30 to 40 in a normal combustion state, and thus a great amount of air (oxygen) remains in exhaust gas after the combustion. This means that a large amount of oxygen is also mixed in the EGR gas, or the exhaust gas.
Therefore, simply increasing the amount of the EGR gas only makes a slow change in the oxygen concentration and the amount of the EGR gas, and thus it may require some time to switch into the low-temperature combustion which is achieved at a high EGR rate. Furthermore, during the low-temperature combustion, the combustion state becomes unstable due to factors such as an increase in the EGR gas, or inert gas, and an engine output may also decrease. To deal with these conditions, various engine controls are used in general so as to reduce a time required for the switching and to secure a combustion stability.
More specifically, in addition to an opening angle control of an EGR valve, an opening angle control of an intake throttle valve, which significantly reduces the amount of air (oxygen) in the intake air by reducing the amount of air relative to the intake air, is carried out. Furthermore, an overshoot control is executed for switching the combustion state at an early stage by once overshooting the amount of the opening angle control of the EGR valve and the intake throttle valve. Meanwhile, a fuel injection system increases and corrects a fuel injection volume, and advances and corrects a fuel injection timing, such that misfire sue to the low-temperature combustion and a decrease in engine output are suppressed.
In the internal combustion engine as described above, switching of combustion state is optimized by performing various engine controls. The aforementioned engine controls are certainly required when returning to the normal combustion state. Processing of such controls are in accordance with the combustion state on each occasion.
One of general fuel injection technologies for diesel engines is a xe2x80x9cpilot injection.xe2x80x9d
The pilot injection injects into a combustion chamber in advance some of the engine fuel to be injected for main injection, so as to create a heat source which serves as a pilot burner for the main combustion, thereby suppressing a sudden increase in a combustion pressure during the main combustion and a rise in the combustion temperature. That is, combustion becomes slow during the execution of the pilot injection, and thus combustion noise can be reduced and emission of harmful components (such as soot and nitrogen oxides NOx) contained in the exhaust gas can be suppressed.
Through the combined use of the low-temperature combustion and the pilot injection, generation of soot (smoke) and emission of nitrogen oxides (NOx) can be suppressed, and at the same time, the combustion noise can be reduced so as to achieve an environment for comfortable driving over the entire operation range.
Meanwhile, according to the inventors and the like of the invention, various improvements were made with respect to the combination of the low-temperature combustion and the pilot injection as mentioned above.
First, if the focus is laid on the low-temperature combustion, combustion becomes slow due to insufficient oxygen and a combustion pressure drop during the low-temperature combustion. As a result, the combustion noise can be reduced and the emission of harmful components can be suppressed as explained above. That is, the same effect obtained with the pilot injection can be obtained during the low-temperature combustion, and thus the pilot injection which has little influence on the engine output results in unnecessary fuel consumption.
Based on the foregoing explanations, it can not always be said that the pilot injection that has generally been executed is an effective fuel injection control technology for all operation ranges.
Furthermore, if the execution or non-execution of the pilot injection is determined upon a request for switching the combustion state, the combustion noise temporarily increases while the combustion state is switched, namely, during transition. That is, if the pilot injection is prohibited despite that the low-temperature combustion state is not yet established, the combustion noise that has been suppressed by the pilot injection is regenerated. Furthermore, if the pilot injection is restarted after the establishment of the normal combustion state, the combustion noise that has been suppressed by the low-temperature combustion temporarily increases during transition from the low-temperature combustion to the normal combustion.
Foregoing descriptions concludes that a key point in the development of an internal combustion engine that uses the low-temperature combustion together with the pilot injection is how to control a prohibited period of the pilot injection in order to achieve both the combustion noise reduction and the pilot injection prohibition.
It is an object of the invention to provide an internal combustion engine provided with an engine control technology capable of regulating pilot injection during low-temperature combustion and suppressing generation of combustion noise caused by prohibition of pilot injection.
To solve the technical problem stated above, the following structure was applied.
That is, an internal combustion engine according to an aspect of the invention has a combustion characteristic in which the amount of soot generated during combustion gradually reaches its peak when a ratio of inert gas contained in an air-fuel mixture subjected to combustion approaches a predetermined value, and if the ratio is further increased, the generation of soot is reduced. The internal combustion engine further includes a control unit for switching between a first combustion state in which generation of soot is restrained by suppressing the ratio of the inert gas below the predetermined value and a second combustion state in which generation of soot is restrained by maintaining the ratio of the inert gas above the predetermined value, and a fuel injection valve which stops execution of, in addition to main fuel injection for the internal combustion engine, the pilot injection which occurs prior to the main fuel injection during a predetermined period at least including a period of the second combustion state.
In the aspect as structured as above, the pilot injection is actively stopped during a period of the second combustion state in which a great amount of EGR gas exists and the combustion becomes slow. That is, the pilot injection is stopped during a period including the second combustion state in which combustion becomes slow, so as to avoid unnecessary fuel consumption due to execution of the pilot injection.
The predetermined period at least including the second combustion state according to the foregoing aspect may be a period defined only by a period of the second combustion state, and also assumes a case in which a part of the first combustion state is included. Furthermore, in the aforementioned aspect, the period of the second combustion state does not necessarily include an entire period of the second combustion state, and may be some period of the entire period of the second combustion state.